1. Field of the Invention
This invention relates generally to catheters and more particularly to catheter apparatus for treating severe or total arterial occlusions. The invention relates especially to the combination of an extravascularly operable imaging and therapeutic device and an intravascular catheter shaft.
2. Background
Atherosclerosis is a disease in which the lumen (interior passage) of an artery becomes stenosed (narrowed) or even totally occluded (blocked) by an accumulation of fibrous, fatty, or calcified tissue. Over time this tissue, known in medicine as an atheroma, hardens and occludes the artery. In the coronary arteries, which supply the heart muscle, this process leads to ischemia (deficient blood flow) of the heart muscle, angina (chest pain), and, eventually, infarction (heart attack) and death. Although drug therapies and modifications to diet and lifestyle show great promise for preventing and treating atherosclerotic vascular disease, many patients urgently require restoration of blood flow that has already been lost, especially in those having severely or totally occluded blood vessels. Unfortunately, the demand for surgical treatment of disabling and life-threatening coronary artery disease will likely increase in the decades ahead.
It has been common surgical practice to treat severe coronary artery disease by performing a coronary bypass, in which a segment of the patient's saphenous vein (taken from the leg) is grafted onto the artery at points upstream and downstream of the stenosis. The bypass often provides dramatic relief. However, this procedure involves not only dangerous open chest surgery, but also an operation on the patient's leg to obtain the segment of saphenous vein that is used for the bypass. Additionally, there is a long, often complicated and painful, convalescence before the patient is healed. Moreover, within a few years, the underlying disease may invade the bypass graft as well. The bypass can be repeated, but at ever greater peril and expense to the patient.
Fortunately, for patients with moderate stenosis, a less traumatic operation is available. A typical mechanical device for such operations is a thin, flexible, tubular device called a catheter. Through a small, conveniently located puncture, the catheter is introduced into a major artery, typically a femoral artery. Under fluoroscopic observation, the catheter is advanced and steered through the arterial system until it enters the stenosed region. At the distal (tip) end of the catheter, a balloon, cutter, or other device dilates the stenosed lumen or removes atheromatous tissue.
Cardiac catheterization procedures for treating stenoses include percutaneous transluminal coronary angioplasty (PTCA), directional coronary atherectomy (DCA), and stenting. PTCA employs a balloon to dilate the stenosis. A steerable guide wire is inserted into and through the stenosis. Next, a balloon-tipped angioplasty catheter is advanced over the guide wire to the stenosis. The balloon is inflated, separating or fracturing the atheroma. Ideally, the lumen will remain patent for a long time. Sometimes, however, it will restenose.
In directional coronary atherectomy, a catheter, containing a cutter housed in its distal end, is advanced over the guide wire into the stenosis. The housing is urged against the atheroma by the inflation of a balloon. Part of the atheroma intrudes through a window in the housing and is shaved away by the cutter.
Stenting is a procedure in which a wire or tubular framework, known as a stent, is compressed onto a balloon catheter and advanced over the guidewire to the stenosis. The balloon is inflated, expanding the stent. Ideally, the stent will hold the arterial lumen open for a prolonged period during which the lumen will remodel itself to a healthy, smooth configuration. Stents are often placed immediately following PTCA or DCA.
It must be noted, however, that a severe stenosis may be untreatable by stenting, DCA, or PTCA. The catheters used in these operations are advanced to their target over a guide wire which has already crossed the stenosis. Most guide wires, however, are too slender and soft-tipped to penetrate the calcified tissue of a severe or total occlusion. Additionally, most guide wires have a bent steering tip which is easily trapped or diverted by the complex, hard tissues often found in a severe stenosis. Without a guidewire to follow, neither PTCA nor DCA nor stenting is feasible and the interventionist may have to refer the patient to bypass surgery. Additionally, degeneration makes a saphenous vein graph a risky and therefore undesirable site of intervention.
Thus, many patients would benefit from a less traumatic alternative to the bypass for restoring circulation after a coronary artery has become severely stenosed or totally occluded. In particular, many such patients would benefit from an operation for crossing the severe or total occlusion without inflicting the gross trauma of classical bypass surgery. In particular, such a procedure is needed for safely forging a path of low mechanical resistance through or around the tough, complex tissues of the severely or totally occlusive atheroma so that blood flow can be restored.
It would be beneficial if a cardiologist could safely cross an occlusion using instruments deliverable by cardiac catheterization or through a small incision in the patient's chest. Instruments have been developed which are deliverable to the site of an occlusion by a cardiac catheter and are capable of penetrating the tissues of a severe or total occlusion or piercing the arterial wall. However, penetrating the arterial wall is what cardiac catheterization procedures usually strive to avoid, because perforation can easily lead to cardiac tamponade. What is needed is a way of reliably selecting the points at which the wall of an artery can be safely penetrated by a catheter working element. What is also needed is a way of reliably guiding the working element from that point of penetration through an occlusion and into another patent portion of the arterial lumen. What is also needed is a way of guiding the working element through interstitial tissue to an appropriately selected point of transvascular entry into an artery.
Likewise, it would be advantageous to reopen the natural lumen of a severely or totally occluded artery so that a stent can be installed or DCA or PTCA can be performed, preferably without straying into the subintimal space or creating a false lumen. Thus, it would be highly advantageous to have a reliable, accurate way of steering and operating a catheter working element along a safe path through the atheromatous tissues of the occlusion.
One guidance system used in coronary catheterization is fluoroscopy, a real-time X-ray technique which is widely used to position devices within the vascular system of a patient. For visualizing a totally occluded artery, biplane fluoroscopy can be used, wherein the interventionist observes two real-time x-ray images acquired from different angles. Biplane fluoroscopy, however, is unreliable, costly and slow.
Another way of imaging the coronary arteries and surrounding tissues is intravascular ultrasound, which employs an ultrasonic transducer in the distal end of a catheter. The catheter may be equipped with an ultraminiature, very high frequency scanning ultrasonic transducer designed to be introduced into the lumen of the diseased artery. Frustratingly, however, the stenosis is often so severe that the transducer will not fit into the part that the interventionist most urgently needs to explore. Indeed, if the occlusion is too severe to be crossed by a guide wire, it may be too difficult to steer the transducer into the segment of greatest interest. Additionally, an attempt to force an imaging catheter into a severely stenosed artery may have undesirable consequences. Alternatively, the intravascular ultrasonic catheter can be placed in a vein adjacent the occluded artery. Because venous lumina are slightly broader than arterial lumina and rarely if ever stenosed, a larger transducer may be employed. Depending on its configuration, a larger transducer may acquire images over greater distances, with finer resolution, or both. However, there is not always a vein properly situated for such imaging.
While superior imaging alone is of diagnostic interest, imaging and guidance for effective intervention for severe occlusive arterial disease is what is truly desired. A reliable imaging technique is needed for discerning precisely the relative positions of a therapeutic working element, the boundaries of the atheromatous tissues of an occlusion, and the structure of the occluded artery as the working element is manipulated.
What is needed is an effective combination of a working element and an imaging system for precisely crossing a severe or total coronary occlusion without causing cardiac tamponade. In particular, such a combination is desired which continuously displays a stable image of the atheroma, the structure of the artery, and the working element as the interventionist maneuvers the working element. What is especially needed is such a combination which is deliverable and operable with minimal trauma to blood vessels and surrounding tissues.